Saw device, saw oscillator, and electronic apparatus

ABSTRACT

A SAW device includes a SAW chip formed of a piezoelectric substrate and an IDT formed thereon, a base substrate that supports the SAW chip, and a fixing member that fixes the SAW chip to the base substrate. The SAW chip that forms a cantilever is supported by the base substrate via the fixing member in a position where the IDT does not overlap with the fixing member in a plan view of the SAW chip. The length W of the SAW chip in a y-axis direction and the length D of the fixing member in the y-axis direction satisfy 1&lt;D/W≦1.6. The fixing member bonds the lower surface and side surfaces of the fixed end of the SAW chip to the base substrate.

BACKGROUND

1. Technical Field

The present invention relates to a SAW device, a SAW oscillator, and anelectronic apparatus.

2. Related Art

A SAW device (surface acoustic wave device) is a circuit element thatconverts an electric signal into a surface wave for signal processingand widely used as a filter, a resonator, and other components. A knownconfiguration of such a SAW device includes a SAW chip formed of apiezoelectric substrate made of quartz or any other piezoelectricmaterial and an IDT electrode (comb electrode) provided on thepiezoelectric substrate and a base substrate to which the SAW chip isfixed with an adhesive (see JP-A-2005-136938, for example).

The SAW device described in JP-A-2005-136938 includes a SAW chip havingan IDT electrode provided on a quartz substrate and a base substratethat supports the SAW chip with an adhesive therebetween. InJP-A-2005-136938, the SAW chip has a mounting portion that is located inone end portion of the SAW chip but does not overlap with the IDTelectrode, and the mounting portion is bonded to the base substrate withan adhesive therebetween, whereby the SAW chip forms a cantileversupported by the base substrate. It is known that supporting the SAWchip as a cantilever as described above improves an aging characteristicof the SAW chip (reduces the amount of time-course variation inoscillating frequency).

Further, in the SAW device described in JP-A-2005-136938, pads formed onthe SAW chip are electrically connected to pads formed on the basesubstrate with wires (bonding wires).

In the thus configured SAW device, the SAW chip needs to be firmlybonded to the base substrate. The firm bonding suppresses unnecessaryvibration of the SAW chip produced when the wires (bonding wires) areconnected to the SAW chip in an ultrasonic bonding process, whereby thewires can be more firmly bonded to the SAW chip.

The SAW device is further preferably so configured that the SAW chip isparallel to the base substrate in order to prevent the SAW chip frombeing broken or fractured. The parallel configuration prevents the freeend of the SAW chip from coming into contact with the base substrate orother components, effectively preventing the SAW chip from being brokenor fractured. The parallel configuration further allows ultrasonicvibration to be efficiently applied to the SAW chip when the wires areconnected to the SAW chip in the ultrasonic bonding process, whereby thewires can be more firmly bonded to the SAW chip.

To meet the requirements described above, it is conceivable to increasethe area where an adhesive is applied onto the SAW chip. In this case,however, the state in which the base substrate supports the SAW chipchanges as the adhesive area increases from “cantilever support” to“entire-area support.” The change in the state degrades the agingcharacteristic of the SAW chip.

To address the problem, in the SAW device descried in JP-A-2005-136938,the SAW chip has a relatively large mounting portion formed in one endportion thereof, and the mounting portion is fixed to the base substratewith an adhesive to achieve a state in which the SAW chip iscantilever-supported by and firmly bonded to the base substrate. In theSAW device described in JP-A-2005-136938, relatively high-level agingcharacteristic, bonding strength, and parallelism are thereforeachieved.

In consideration of characteristics and precision of a SAW device thatare required in recent years and in the future, however, the SAW devicedescribed in JP-A-2005-136938 is insufficient in terms of therequirements described above (bonding strength and parallelism, inparticular) and hence required to be improved.

SUMMARY

An advantage of some aspects of the invention is to provide a SAWdevice, a SAW oscillator, and an electronic apparatus that achievehigh-level aging characteristic, bonding strength, and parallelism.

Application Example 1

A SAW device according to this application example of the inventionincludes a SAW chip including a plate-shaped piezoelectric substrate anda comb electrode disposed on the piezoelectric substrate, abasesubstrate on which the SAW chip is mounted, and a fixing member thatfixes the SAW chip to the base substrate and supports the SAW that formsa cantilever in a position where the comb electrode does not overlapwith the fixing member in a plan view of the SAW chip, wherein the combelectrode is disposed on a surface of the piezoelectric substrate thatfaces away from the base substrate, a length W of the SAW chip in afirst direction and a length D of the fixing member in the firstdirection satisfy 1<D/W≦1.6, the first direction being perpendicular toa direction in which a fixed end and a free end of the SAW chip areapart from each other in a plan view of the base substrate, and thefixing member bonds a surface of the fixed end of the SAW chip thatfaces the base substrate and a pair of side surfaces of the fixed end ofthe SAW chip that face each other in the first direction to the basesubstrate.

According to this application example, a SAW device having a high-levelaging characteristic, bonding strength, and parallelism can be provided.Specifically, variation in frequency of the SAW device having beendriven continuously for 10 years in an atmosphere of a room temperature(25° C.)±20° C. falls within ±10 ppm.

Application Example 2

In the SAW device according to the application example, it is preferablethat the piezoelectric substrate is made of quartz.

According to this application example, excellent temperature andfrequency characteristics are achieved.

Application Example 3

In the SAW device according to the application example, it is preferablethat the fixed end of the piezoelectric substrate has a thickness t, andthe fixing member reaches a height greater than or equal to 0.2 t butsmaller than or equal to 0.8 t measured from base substrate-side ends ofthe side surfaces of the fixed end.

According to this application example, the SAW chip can be firmly fixedto the base substrate, and decrease in the aging characteristic can beprevented.

Application Example 4

In the SAW device according to the application example, it is preferablethat the fixing member extends off outward in a direction from the freeend of the SAW chip toward the fixed end thereof and outward from bothsides of the SAW chip in the first direction in the plan view of thebase substrate.

According to this application example, the SAW chip can be firmly fixedto the base substrate.

Application Example 5

In the SAW device according to the application example, the contour ofthe fixing member extends along the contour of the fixed end of the SAWchip in the plan view of the base substrate.

According to this application example, the SAW chip can be firmly fixedto the base substrate. In addition to this, the fixing member will notextend off excessively from the fixed end, whereby the volume of thefixing member can be reduced.

Application Example 6

In the SAW device according to the application example, it is preferablethat the SAW chip is disposed in parallel to the base substrate.

According to this application example, the free end of the SAW chip willnot come into contact with the base substrate, whereby the SAW chip willnot be broken or fractured, and the reliability of the SAW device isimproved.

Application Example 7

In the SAW device according to the application example, the SAW chip hasa connection pad provided on a surface that faces away from the basesubstrate in a position above and within the fixing member in the planview of the SAW chip.

According to this application example, a metal wire can be firmly bondedto the pad.

Application Example 8

In the SAW device according to the application example, it is preferablethat the Young's modulus of the fixing member is greater than or equalto 0.02 GPa but smaller than or equal to 4 GPa.

According to this application example, the SAW chip can be firmly andstably fixed to the base substrate.

Application Example 9

A SAW oscillator according to this application example of the inventionincludes any of the SAW devices according to the application examplesdescribed above, and an oscillation circuit that applies a voltage tothe comb electrode to oscillate the SAW chip.

According to this application example, a reliable SAW oscillator isprovided.

Application Example 10

An electronic apparatus according to this application example of theinvention includes any of the SAW devices according to the applicationexamples described above.

According to this application example, a reliable electronic apparatusis provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a plan view (top view) showing a SAW device according to afirst embodiment of invention.

FIG. 2 is a cross-sectional view of the SAW device shown in FIG. 1 takenalong the line A-A.

FIG. 3 is a cross-sectional view of the SAW device shown in FIG. 1 takenalong the line B-B.

FIGS. 4A and 4B are plan views showing variations of a fixing memberprovided in the SAW device shown in FIG. 1.

FIG. 5 shows a graph representing the relationship between D/W and ΔF/F(AVG).

FIG. 6 shows a graph representing standard deviation σ of variation inΔF/F.

FIG. 7 shows graphs representing [ΔF/F (AVG)+3σ] and [ΔF/F (AVG)−3σ].

FIG. 8 is a plan view (top view) of a SAW device according to a secondembodiment of invention.

FIG. 9 is a plan view (top view) of a SAW oscillator according to anembodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A SAW device, a SAW oscillator, and an electronic apparatus according toembodiments of the invention will be described below in detail withreference to the accompanying drawings.

1. SAW Device

First Embodiment

A SAW device according to a first embodiment of the invention will firstbe described.

FIG. 1 is a plan view (top view) showing the SAW device according to thefirst embodiment of invention. FIG. 2 is a cross-sectional view of theSAW device shown in FIG. 1 taken along the line A-A. FIG. 3 is across-sectional view of the SAW device shown in FIG. 1 taken along theline B-B. In the following description, directional definitions are madewith respect to the plane of view of FIG. 1 for the convenience ofdescription: “up” refers to the direction oriented toward the reader;“down” refers to the direction oriented away from the reader; “left”refers to the direction oriented leftward; and “right” refers to thedirection oriented rightward. Further, three axes perpendicular to oneanother are called x, y, and z axes, and the z axis coincides with thethickness direction of the SAW device (SAW chip), as shown in FIG. 1.Further, the direction parallel to the x axis is called an “x-axisdirection,” the direction parallel to the y axis is called a “y-axisdirection,” and the direction parallel to the z axis is called a “z-axisdirection.” In FIG. 1, a lid 32, which will be described later, isomitted.

A SAW device (surface acoustic wave device) 1 shown in FIG. 1 includes aSAW chip (surface acoustic wave element) 2, a package 3 thataccommodates the SAW chip 2, and a fixing member 4 that fixes the SAWchip 2 to the package 3. The SAW device 1, which includes the SAW chip2, can form a SAW resonator, a SAW oscillator, and other SAW-relateddevices.

The components described above will be sequentially described below indetail.

Package 3

The package 3 includes a package base 31 having a recess open upward anda lid 32 so bonded to the package base 31 that the lid 32 covers therecess, as shown in FIGS. 1 to 3. The package 3 has an accommodationportion 33 surrounded by the package base 31 and the lid 32, and the SAWchip 2 is so accommodated in the accommodation portion 33 that the SAWchip 2 is not in contact with the package 3. The interior of theaccommodation portion 33 is preferably maintained under vacuum or filledwith nitrogen.

The package base 31 is formed of a plate-shaped base substrate 311 and aframe-shaped sidewall 312 provided along the periphery of the uppersurface of the base substrate 311. The base substrate 311 extends in thexy plane and has a thickness in the z-axis direction.

A pair of connection pads 81 and 82 are provided on the upper surface ofthe base substrate 311. The connection pads 81 and 82 are electricallyconnected to bonding pads 251 and 252, which belong to the SAW chip 2,with metal wires (bonding wires) 51 and 52 made, for example, of gold.

The package base 31 is preferably made of an insulating (non-conductive)material, for example, an aluminum oxide or any other variety ofceramics.

The lid 32 is not necessarily made of a specific material and may bemade of a material whose coefficient of linear thermal expansion issimilar to that of the material of which the package base 31 is made.For example, when the package base 31 is made of the ceramic describedabove, the lid 32 is preferably made of kovar or any other similaralloy. The lid 32 is bonded to the package base 31, for example, with aseal ring (not shown) therebetween in a seam welding process.

SAW Chip 2

The SAW chip 2 includes a plate-shaped piezoelectric substrate 21, anIDT (comb electrode) 22 provided on the upper surface of thepiezoelectric substrate 21, a pair of reflectors 231 and 232 disposed onopposite sides of the IDT 22, extracting electrodes 241 and 242electrically connected to the IDT 22, and bonding pads (pads) 251 and252 electrically connected to the extracting electrodes 241 and 242, asshown in FIG. 1.

The piezoelectric substrate 21 extends in the xy plane and has athickness in the z-axis direction. The piezoelectric substrate 21 in aplan view has a substantially rectangular shape whose longitudinaldirection coincides with the x-axis direction.

The thus shaped piezoelectric substrate 21 is made of quartz. Thepiezoelectric substrate 21 made of quartz provides excellent temperatureand frequency characteristics. The piezoelectric substrate 21 mayalternatively be made, for example, of lithium tantalate, lithiumniobate, lithium borate, or any other suitable piezoelectric materialother than quartz.

The IDT 22 is disposed on the piezoelectric substrate 21 in a centralportion thereof in the x-axis direction. The IDT 22 is formed of a pairof electrodes 221 and 222. The pair of electrodes 221 and 222 are sodisposed that electrode fingers of the electrode 221 are interleavedwith electrode fingers of the electrode 222.

When a voltage is applied between the pair of electrodes 221 and 222,the piezoelectric effect of the piezoelectric substrate 21 producescyclic strain between the electrode fingers, and the cyclic straininduces a surface acoustic wave in the piezoelectric substrate 21. Theinduced surface acoustic wave propagates along the direction in whichthe electrode fingers are arranged (x-axis direction).

The pair of reflectors 231 and 232 are disposed on opposite sides of theIDT 22 in the direction in which the surface acoustic wave describedabove propagates (x-axis direction). The reflectors 231 and 232 have afunction of reflecting the surface acoustic wave that propagates alongthe piezoelectric substrate 21 and containing the surface acoustic wavebetween the reflectors 231 and 232. Only one IDT is disposed between thereflectors 231 and 232 in FIG. 1, but the number of IDTs may be two ormore.

The IDT 22 and the reflectors 231 and 232 are as a whole shifted towardone end of the piezoelectric substrate 21 in the longitudinal directionthereof as shown in FIG. 1 (shifted toward right end in FIG. 1). Thepair of bonding pads 251 and 252 are formed on the upper surface on theother side of the piezoelectric substrate 21 (shifted to left end inFIG. 1). Further, the extracting electrodes 241 and 242 are formed onthe upper surface of the piezoelectric substrate 21. The bonding pad 251is electrically connected to the electrode 221 via the extractingelectrode 241, and the bonding pad 252 is electrically connected to theelectrode 222 via the extracting electrode 242.

As described above, the bonding pad 251 is electrically connected to theconnection pad 81 via the metal wire 51, and the bonding pad 252 iselectrically connected to the connection pad 82 via the metal wire 52.

The IDT 22, the reflectors 231 and 232, the extracting electrodes 241and 242, and the bonding pads 251 and 252 can all be made of aluminum,an aluminum alloy, or any other metal material having excellentelectrical conductivity.

The configuration of the SAW chip 2 has been described.

The thus configured SAW chip 2, specifically, the one end portionthereof in the longitudinal direction (left end portion in FIG. 1), isfixed (bonded) to the base substrate 311 via the fixing member 4, asshown in FIG. 1. That is, the base substrate 311 supports the SAW chip 2as a cantilever having left and right ends in FIG. 1 as a fixed end 28and a free end 29 respectively. Supporting the SAW chip 2 as acantilever prevents deformation of the SAW chip 2 caused by an externalforce or thermal stress, whereby it is possible to effectively preventthe frequency characteristic of the SAW chip 2 from changing ordegrading.

The SAW chip 2 is further so fixed to the base substrate 311 via thefixing member 4 in parallel to the base substrate 311, as shown in FIGS.2 and 3. Maintaining the SAW chip 2 parallel to the base substrate 311prevents the free end 29 of the SAW chip 2 from coming into contact withthe base substrate 311 and hence the SAW chip 2 from being broken orfractured, whereby the reliability of the SAW device 1 is improved.

Further, the bonding strength between the bonding pads 251 and 252 andthe metal wires 51 and 51 can be increased, whereby the reliability ofthe SAW device 1 is improved also in this regard. Specifically, themetal wire 51 is bonded to the bonding pad 251 in a known wire bondingprocess. The wire bonding process includes the steps of applying a highvoltage to the tip of the metal wire 51, which has been allowed toprotrude from the tip of a capillary, to form a ball (FAB), pressing thetip of the metal wire 51 against the bonding pad 251 downward in thez-axis direction, and applying ultrasonic vibration from the capillaryto the bonding pad 251 to connect the metal wire 51 to the bonding pad251 in a thermo-compression bonding manner. In the thus performedprocess, an oxide film on the surface of the bonding pad 251 is broken,and the metal wire 51 is firmly bonded to a newly formed surface of thebonding pad 251 to form a bonding interface, whereby the metal wire 51can be firmly bonded to the bonding pad 251.

When the SAW chip 2 is parallel to the base substrate 311 in the wirebonding process described above, the bonding pad 251 is perpendicular tothe z axis. In this case, the tip of the metal wire 51 can beperpendicularly pressed against the bonding pad 251 downward in thez-axis direction, whereby the pressing force at which the tip is pressedagainst the bonding pad can be large enough and the capillary canefficiently apply ultrasonic vibration to the bonding pad 251. As aresult, the metal wire 51 can be more firmly bonded to the bonding pad251, and the reliability of the SAW device 1 is improved accordingly.

In particular, the bonding pad 251 and 252 are so formed that theyoverlie the fixing member 4, in other words, they are located within thefixing member 4 in the plan view (xy-plane view), as shown in FIG. 1.Since the bonding pads 251 and 252 are supported by the fixing member 4,which is located immediately below the bonding pads 251 and 252 asdescribed above, the ultrasonic vibration applied from the capillarywill not leak elsewhere, whereby the metal wires 51 and 52 can be morefirmly bonded to the bonding pads 251 and 252.

Further, the SAW chip 2 is fixed to the fixing member 4 with the surfaceof the SAW chip 2 on which the IDT 22 is disposed facing upward, asshown in FIG. 1. The configuration effectively prevents the IDT 22 fromcoming into contact with the fixing member 4, whereby it is possible toeffectively prevent the aging characteristic of the SAW device 1 fromdegrading.

Fixing Member 4

The fixing member 4 is disposed between the SAW chip 2 and the basesubstrate 311 and fixes the SAW chip 2 to the base substrate 311, asshown in FIGS. 1 to 3. The thus configured fixing member 4 is notnecessarily made of a specific material and can be made, for example, ofa silicon-based adhesive, an epoxy-based adhesive, a polyimide-basedadhesive, or any other variety of adhesives capable of fixing the SAWchip 2 to the base substrate 311.

Fixing the SAW chip 2 to the base substrate 311 via the fixing member 4includes, for example, the steps of applying the fixing member 4 that isnot yet hardened to the upper surface of the base substrate 311, placingthe SAW chip 2 on the fixing member 4, lightly pressing the SAW chip 2against the fixing member 4, and heating the fixing member 4 to apredetermined temperature to harden the fixing member 4.

The Young's modulus of the fixing member 4 is not limited to a specificvalue but is preferably greater than or equal to 0.02 GPa but smallerthan or equal to 4 GPa. The Young's modulus within the range describedabove allows the SAW chip 2 to be firmly and stably fixed to the basesubstrate 311. When the Young's modulus is smaller than the lower limitdescribed above, the leakage of the ultrasonic vibration from the SAWchip 2 increases in the wire bonding process described above althoughthe amount of leakage depends on the shape, volume, and other factors ofthe fixing member 4. In this case, the bonding strength between thebonding pads 251, 252 and the metal wires 51, 52 can bedisadvantageously insufficient. On the other hand, when the Young'smodulus is greater than the upper limit described above, and the fixingmember 4 that has hardened and contracted distorts the SAW chip 2(piezoelectric substrate 21) although the amount of distortion dependson the shape, volume, and other factors of the fixing member 4, and thefrequency characteristic of the SAW chip 2 can be disadvantageouslychanged or degraded.

The fixing member 4 is preferably made of an insulating material not tocause short circuit or any other electric failure. The fixing member 4may alternatively be electrically conductive unless the fixing member 4comes into contact with the connection pads 81 and 82 formed on the basesubstrate 311, the bonding pads 251 and 252, which belong to the SAWchip 2, or other pads and causes short circuit.

The thus configured fixing member 4 protrudes (extends off) outward fromthe entire circumference of the fixed end 28 of the SAW chip 2 in the xyplan view, and the protruding portion of the fixing member 4 is exposed,as shown in FIG. 1. The fixing member 4 is further so formed that itextends around from a lower surface 281 of the fixed end 28 of the SAWchip 2 to side surfaces 282 thereof and fixes the SAW chip 2,specifically, the lower surface 281 and the side surfaces 282 of thefixed end 28 of the SAW chip 2, to the base substrate 311, as shown inFIGS. 2 and 3. The SAW chip 2 can therefore be firmly fixed to the basesubstrate 311. Specifically, in the SAW device 1, fixing member 4 notonly upwardly supports the SAW chip 2 but also supports the SAW chip 2in the y-axis direction by sandwiching both sides thereof. The SAW chip2 can therefore be fixed to the base substrate 311 more stably andfirmly than in a case where a SAW chip is fixed to a base substrate onlyby using the lower surface of the SAW chip in related art.

A first advantage of the configuration described above is that leakageof the ultrasonic vibration from the SAW chip 2 is reduced in the wirebonding process described above, whereby the metal wires 51 and 52 canbe firmly bonded to the bonding pads 251 and 252. A second advantage isthat since the bonding strength between the SAW chip 2 and the basesubstrate 311 increases, high-level parallelism of the SAW chip 2 withthe base substrate 311 can be readily achieved. As a result, theadvantageous effect described above can be reliably provided.

In particular, in the present embodiment, the fixing member 4 is soformed that it protrudes outward from the entire circumference of thefixed end 28 of the SAW chip 2, as described above. Not only a pair ofside surfaces 282 a and 282 b of the fixed end 28 that face each otherin the y-axis direction but also a side surface 282 c that connects theside surfaces 282 a and 282 b to each other are therefore fixed to thebase substrate 311 via the fixing member 4. As a result, the SAW chip 2can be more firmly fixed to the base substrate 311, and the advantageouseffect described above becomes more pronounced.

Further, the shape of the contour of a portion 4 a of the fixing member4 that is exposed from the SAW chip 2 corresponds to the shape of thecontour of the fixed end 28 of the SAW chip 2 in the xy plan view, asshown in FIG. 1. In other words, the amount of protrusion (protrudinglength) of the fixing member 4 from the fixed end 28 is substantiallythe same along the entire circumference of the fixed end 28. The thusshaped fixing member 4 allows the SAW chip 2 to be firmly fixed to thebase substrate 311. In addition to this, since the fixing member 4 doesnot excessively extend off from the fixed end 28, the volume of thefixing member 4 can be reduced, which can reduce the amount of gasproduced (outgassed) from the fixing member 4, whereby degradation inthe aging characteristic due to the outgassing can be effectivelyreduced.

It is further preferable that the fixing member 4 is not present on theupper surface of the SAW chip 2. In other words, in the SAW device 1, itis preferable that the fixing member 4, which extends around to the sidesurfaces 282 of the fixed end 28 of the SAW chip 2, does not extendaround to the upper surface thereof. When the fixing member 4, if itextends around to the upper surface of the SAW chip 2, comes intocontact with the IDT 22, which is formed on the upper surface of the SAWchip 2, the aging characteristic of the SAW device 1 is degraded. Thefixing member 4 is therefore preferably so formed that it does notextend around to the upper surface of the SAW chip 2.

Further, the fixing member 4 is so formed that it extends around to theside surfaces 282 of the fixed end 28 of the SAW chip 2, as describedabove. Now, let t be the thickness of the SAW chip 2 (piezoelectricsubstrate 21), as shown in FIG. 3. The fixing member 4 preferablyreaches a height greater than or equal to 0.2 t measured from the lowerends of the side surfaces 282 of the fixed end 28. The fixing member 4that reaches the height described above more reliably provides theadvantageous effect described above, which is “the SAW chip 2 can befirmly fixed to the base substrate 311.”

Further, the fixing member 4 preferably reaches a height of 1.0 tmeasured from the lower ends of the side surfaces 282 of the fixed end28. That is, the fixing member 4 is preferably formed all over the sidesurfaces 282 of the fixed end 28 in the thickness direction. The thusshaped fixing member 4 makes the advantageous effect described abovemore pronounced.

The fixing member 4 formed all over the side surfaces 282 of the fixedend 28 in the thickness direction, however, tends to extend around fromthe side surfaces 282 of the fixed end 28 of the SAW chip 2 to the uppersurface of the fixed end 28. In this case, the fixing member 4 can comeinto contact with the IDT 22, which can disadvantageously degrade thefrequency and other characteristics of the SAW chip 2. The productivityand yield of the SAW device 1 can therefore decrease when the fixingmember 4 is formed all over the side surfaces 282 of the fixed end 28 inthe thickness direction. To solve the problem, the fixing member 4preferably reaches a height smaller than or equal to 0.8 t measured fromthe lower ends of the side surfaces 282 of the fixed end 28.

That is, the fixing member 4 preferably reaches a height greater than orequal to 0.2 t measured from the lower ends of the side surface 282 ofthe fixed end 28 but smaller than or equal to 0.8 t thereof. The thusshaped fixing member 4 not only allows the SAW chip 2 to be firmly fixedto the base substrate 311 but also prevents decrease in productivity andyield of the SAW device 1.

Now, let W be the length of the fixed end 28 of the SAW chip 2 in they-axis direction (direction perpendicular to the x-axis direction, whichis the direction in which the fixed end 28 and the free end 29 are apartfrom each other (first direction) in the xy plan view), and let D be thelength of the fixing member 4 in the y-axis direction. The SAW device 1is so configured that 1<D/W≦1.6 is satisfied. The length W is notlimited to a specific value but ranges, for example, from about 0.5 to2.0 mm. Further, the fixing member 4, which is made of an adhesive, inmany cases does not have a perfect rectangular shape but has asubstantially rectangular shape with the outermost circumferentialportion waved when the adhesive is applied and dried. In this case, Drepresents the greatest length of the fixing member in the y-axisdirection.

When the relationship described above is satisfied, the SAW device 1 hasan excellent aging characteristic. That is, the amount of time-coursevariation in oscillation frequency (resonant frequency) of the SAWdevice 1 can be reduced.

The acceptable amount of time-course variation in oscillation frequencyof the SAW device 1 is not limited to a specific value, but the amountof variation in oscillation frequency at a point somewhere after the SAWdevice 1 has been continuously driven for 10 years in a room temperature(25° C.) atmosphere preferably falls within a ±10 ppm. Since theoscillation frequency of the SAW device 1 gradually changes with time,satisfying the condition described above means that maintaining theoscillation frequency of the SAW device 1 within ±10 ppm of theoscillation frequency before the aging for 10 years. The SAW device 1that satisfies the condition can be used suitably as a referenceoscillation source, for example, in a wireless base station where ahigh-level aging characteristic is required.

When D/W is smaller than or equal to the lower limit described above,the fixing member 4 cannot extend around to the side surfaces 282 of theSAW chip 2. In this case, the SAW chip 2 cannot be fixed stably orfirmly to the base substrate 311. On the other hand, when D/W is greaterthan the upper limit described above, the volume (amount) of the fixingmember 4 becomes too large. In this case, substances resulting fromoutgassing and other reactions in the fixing member 4 degrades the agingcharacteristic of the SAW device 1 (increases time-course variation inoscillation frequency).

FIG. 5 shows a graph representing the relationship between D/W and ΔF/F(AVG). ΔF/F (AVG) represents the average of the amounts of variationΔF/F in the oscillation frequencies of 22 samples of the SAW device 1 ata point somewhere after aging. ΔF/F can be expressed by (F₁−F₀)/F₀,where F₀ represents the oscillation frequency of a sample before aging,and F₁ represents the oscillation frequency of the sample after aging.The aging was performed in a 125° C. atmosphere for a continuous driveperiod of 1000 hours.

FIG. 6 shows a graph representing standard deviation σ of the variationin ΔF/F of the 22 samples described above. That is, σ=0 means that ΔF/Fdoes not vary among the samples, whereas a large σ means that ΔF/Fgreatly varies among the samples.

FIG. 7 shows a graph obtained by plotting ΔF/F (AVG) shown in FIG. 5 towhich σ shown in FIG. 6 multiplied by three is added and a graphobtained by plotting ΔF/F (AVG) shown in FIG. 5 from which σ shown inFIG. 6 multiplied by three is subtracted, that is, graphs obtained byplotting [ΔF/F (AVG)+3σ] and [ΔF/F (AVG)−3σ].

When the SAW device 1 is manufactured in volume, at least 99.7% of theSAW devices 1, that is, most of the volume-manufactured SAW devices 1,have ΔF/F that falls within ΔF/F (AVG)±3σ. Further, the aging conditiondescribed above, “a 125° C.-atmosphere for continuous drive period of1000 hours,” corresponds to a 5° C.-atmosphere for continuous driveperiod of 300 years and a 45° C.-atmosphere for continuous drive periodof 12 years.

In view of the fact described above, when the relationship of −10ppm≦ΔF/F (AVG)±3σ≦10 ppm is satisfied, variation in frequency of almostall the manufactured SAW devices 1 having been driven continuously for10 years in an atmosphere of room temperature (25° C.)±20° C. fallswithin ±10 ppm. As a result, the thus manufactured SAW devices 1 can besuitably used as a reference oscillation source, for example, in awireless base station as described above. To satisfy −10 ppm≦ΔF/F(AVG)±3σ≦10 ppm, D/W may be set at 1.6 or smaller, as shown in FIG. 7.

It is noted that the lower limit of D/W is not limited to a specificvalue in consideration only of the aging characteristic, as shown inFIG. 7. When D/W is lower than or equal to 1, however, the SAW chip 2cannot disadvantageously be stably or firmly fixed to the base substrate311, as described above. To fix the SAW chip 2 stably and firmly to thebase substrate 311 and provide an excellent aging characteristic, theSAW device 1 is therefore configured to satisfy 1<D/W≦1.6.

The SAW chip 2 in the SAW device 1 used as any of the samples describedabove had an xy-plane dimension of about 3.2×0.9 mm and a thickness ofabout 0.5 mm. Further, the fixing member 4 had a substantiallyrectangular shape having a length of about 1.0 mm in the X direction anda thickness of about 0.9 mm, and the fixing member 4 reached a height ofabout 0.6 t measured from the ends of the side surfaces 282 of the fixedend 28 that face the base substrate 311. Further, the fixing member 4,which had a substantially rectangular shape, had an outermost peripheralshape that was waved within a range of about 0.1 mm, and the length D ofthe fixing member 4 was about 1.1 mm, which was the greatest length ofthe contour thereof in the Y direction.

The SAW device 1 has been described above. According to the thusconfigured SAW device 1, the aging characteristic of the SAW chip 2, thebonding strength between the metal wires 51 and 52 and the SAW chip 2,and the parallelism of the SAW chip 2 with the base substrate 311 can bemaintained at high levels.

In the SAW device 1 according to the present embodiment, the fixingmember 4 protrudes from the entire circumference of the fixed end 28 ofthe SAW chip 2. The fixing member 4 may alternatively protrude at leastfrom the side surfaces 282 a and 282 b of the fixed end 28 that faceeach other in the y-axis direction. Specifically, for example, thefixing member 4 may have an elliptical shape elongated in the y-axisdirection and protrude from the side surfaces 282 a and 282 b but maynot protrude from the side surface 282 c, as shown in FIG. 4B.

Further, in the SAW device 1 according to the present embodiment, theshape of the contour of the fixing member 4 corresponds to the shape ofthe contour of the fixed end 28 of the SAW chip 2. The shape of thecontour of the fixing member 4 does not necessarily correspond to theshape of the contour of the fixed end 28 of the SAW chip 2.Specifically, the fixing member 4 may, for example, have a circularshape as shown in FIG. 4A or a different shape.

Second Embodiment

A SAW device according to a second embodiment of the invention will nextbe described.

FIG. 8 is a plan view (top view) of the SAW device according to thesecond embodiment of invention. In FIG. 8, a lid 32 is omitted forconvenience of description.

The SAW device according to the second embodiment will be describedbelow primarily about differences from the embodiment described above,and no description of the same items will be made.

The SAW device according to the second embodiment of the invention issubstantially the same as the SAW device according to the firstembodiment described above and only differs therefrom in terms of theconfiguration of the SAW chip. The same components as those in the firstembodiment described above have the same reference characters.

A SAW chip 2 provided in a SAW device 1 according to the presentembodiment has an IDT 22 formed on a piezoelectric substrate 21 in aright-side portion thereof and in a central portion thereof in they-axis direction in FIG. 8 and a pair of reflectors 231 and 232 formedon opposite sides of the IDT 22 in the y-axis direction, as shown inFIG. 8. In the thus configured SAW chip 2, an induced surface acousticwave propagates in the y-axis direction.

The SAW chip 2, specifically, a left-side portion thereof in FIG. 8, isfixed to a base substrate 311 via a fixing member 4. That is, the SAWchip 2 is so fixed to the base substrate 311 that the left end of theSAW chip 2 in FIG. 8 is a fixed end 28 and the right end of the SAW chip2 in FIG. 8 is a free end 29.

In the thus configured second embodiment, the same advantageous effectas that provided in the first embodiment described above is provided.

2. SAW Oscillator

A SAW oscillator into which any of the SAW devices 1 described above isincorporated (SAW oscillator according to an embodiment of theinvention) will next be described.

A SAW oscillator 100 includes any of the SAW devices 1 and an IC chip 9,as shown in FIG. 9. The IC chip 9 is provided in the accommodationportion 33 and so fixed to the base substrate 311 that the IC chip 9 andthe SAW chip 2 are disposed side by side. The thus configured IC chip 9is electrically connected to the SAW chip 2 via connection pads 81 and82, and an oscillation circuit (circuit that oscillates SAW chip 2)built in the IC chip 9 can oscillate the SAW chip 2.

3. Electronic Apparatus

The SAW device 1 described above can be incorporated into a variety ofelectronic apparatus. The electronic apparatus into which the SAW device1 is incorporated is not limited to a specific one and may include apersonal computer (mobile personal computer, for example); a mobilephone and any other portable terminal; a digital still camera; an inkjetdischarger (inkjet printer, for example), a laptop personal computer, atablet personal computer, a router, a switch, and other storage areanetwork apparatus; a local-area network apparatus, a televisionreceiver, a video camcorder, a video tape recorder, a car navigationsystem, a pager, an electronic organizer (including one withcommunication capability), an electronic dictionary, a desktopcalculator, an electronic game console, a game controller, a wordprocessor, a workstation, a TV phone, a security TV monitor, electronicbinoculars, a POS terminal, a medical apparatus (electronic clinicalthermometer, blood-pressure gauge, blood-sugar gauge,electrocardiograph, ultrasonic diagnostic apparatus, and electronicendoscope, for example), a fish finder, a variety of measurementapparatus, a variety of instruments (instruments for vehicle, airplane,and ship), a flight simulator, a head-mount display, a motion tracer, amotion tracker, a motion controller, and a PDR (pedestrian deadreckoning) apparatus.

The SAW device, the SAW oscillator, and the electronic apparatusaccording to the illustrated embodiments of the invention have beendescribed above, but the invention is not limited thereto. Theconfiguration of each of the components can be replaced with anarbitrary configuration that has the same function. Further, otherarbitrary configurations and steps may be added. Moreover, the SAWdevice, the SAW oscillator, and the electronic apparatus according toembodiments of the invention may be combinations of at least arbitrarytwo configurations (features) of the embodiments described above.

The above embodiments have been described with reference to theconfiguration in which the SAW chip is fixed to the base substrate inparallel thereto, but the SAW chip may alternatively be inclined to thebase substrate. Specifically, the free end may be so inclined to thebase substrate that it is set apart from the base substrate by a greateramount than the fixed end. Conversely, the free end may be so inclinedto the base substrate that it is set apart from the base substrate by asmaller amount than the fixed end.

The entire disclosure of Japanese Patent Application No. 2011-165798,filed Jul. 28, 2011 is expressly incorporated by reference herein.

What is claimed is:
 1. A SAW device comprising: a SAW chip including aplate-shaped piezoelectric substrate and a comb electrode disposed onthe piezoelectric substrate; a base substrate on which the SAW chip ismounted; and a fixing member that fixes the SAW chip to the basesubstrate and supports the SAW chip that forms a cantilever in aposition where the comb electrode does not overlap with the fixingmember in a plan view of the SAW chip, wherein the comb electrode isdisposed on a surface of the piezoelectric substrate that faces awayfrom the base substrate, a length W of the SAW chip in a first directionand a length D of the fixing member in the first direction satisfy1<D/W≦1.6, the first direction being perpendicular to a direction inwhich a fixed end and a free end of the SAW chip are apart from eachother in a plan view of the base substrate, and the fixing member bondsa surface of the fixed end of the SAW chip that faces the base substrateand a pair of side surfaces of the fixed end of the SAW chip that faceeach other in the first direction to the base substrate.
 2. The SAWdevice according to claim 1, wherein the piezoelectric substrate is madeof quartz.
 3. A SAW oscillator comprising: the SAW device according toclaim 2; and an oscillation circuit that applies a voltage to the combelectrode to oscillate the SAW chip.
 4. An electronic apparatuscomprising the SAW device according to claim
 2. 5. The SAW deviceaccording to claim 1, wherein the fixed end of the piezoelectricsubstrate has a thickness t, and the fixing member reaches a heightgreater than or equal to 0.2 t but smaller than or equal to 0.8 tmeasured from base substrate-side ends of the side surfaces of the fixedend.
 6. A SAW oscillator comprising: the SAW device according to claim3; and an oscillation circuit that applies a voltage to the combelectrode to oscillate the SAW chip.
 7. An electronic apparatuscomprising the SAW device according to claim
 3. 8. The SAW deviceaccording to claim 1, wherein the fixing member extends off outward in adirection from the free end of the SAW chip toward the fixed end thereofand outward from both sides of the SAW chip in the first direction inthe plan view of the base substrate.
 9. A SAW oscillator comprising: theSAW device according to claim 4; and an oscillation circuit that appliesa voltage to the comb electrode to oscillate the SAW chip.
 10. Anelectronic apparatus comprising the SAW device according to claim
 4. 11.The SAW device according to claim 1, wherein the contour of the fixingmember extends along the contour of the fixed end of the SAW chip in theplan view of the base substrate.
 12. A SAW oscillator comprising: theSAW device according to claim 5; and an oscillation circuit that appliesa voltage to the comb electrode to oscillate the SAW chip.
 13. Anelectronic apparatus comprising the SAW device according to claim
 5. 14.The SAW device according to claim 1, wherein the SAW chip is disposed inparallel to the base substrate.
 15. A SAW oscillator comprising: the SAWdevice according to claim 6; and an oscillation circuit that applies avoltage to the comb electrode to oscillate the SAW chip.
 16. Anelectronic apparatus comprising the SAW device according to claim
 6. 17.The SAW device according to claim 1, wherein the SAW chip has aconnection pad provided on a surface that faces away from the basesubstrate in a position above and within the fixing member in the planview of the SAW chip.
 18. The SAW device according to claim 1, whereinthe Young's modulus of the fixing member is greater than or equal to0.02 GPa but smaller than or equal to 4 GPa.
 19. A SAW oscillatorcomprising: the SAW device according to claim 1; and an oscillationcircuit that applies a voltage to the comb electrode to oscillate theSAW chip.
 20. An electronic apparatus comprising the SAW deviceaccording to claim 1.